how can I make a 3D object appear behind an object in a video
There are a couple of ways you can do this, off the top of my head:

1: MASKING - you can setup a simple box mask to remove/hide the part of the 3D object so that it appears to be "hidden" partially behind the wall in the video even though it is a layer sitting on top of the video.

2: PROJECTION MAPPING - this way is more complicated. You recreate the video scene in the 3D viewport, apply the video as textures to each object, set lighting, then position your 3D object and animate as necessary. This gives more accurate results in terms of shadows from the 3D animated object moving within the actual/recreated scene.

Depending on the end result that you're looking for, these are just a couple of ways to get there.